1. Field of the Invention
The present invention relates to a hydraulic control system for automatic transmissions, and more particularly, to a hydraulic control system for automatic transmissions which can alleviate shift shock when shifting from a fourth speed to a third speed in a drive D range and when shifting from a neutral N range to a reverse R range.
2. Description of the Related Art
Conventional automatic transmissions used in vehicles include a torque converter, a multi-stage gear shift mechanism connected to the torque converter, and a plurality of friction members actuated by hydraulic pressure for selecting one of the gear stages of the gear shift mechanism, the gear shift mechanism being realized through a planetary gearset. The friction elements are controlled to engaged and disengaged states by a hydraulic control system, which controls pressure generated in a hydraulic pump, to change shift ratios of the planetary gearset.
The friction elements are selectively operated by a plurality of valves, which undergo port conversion to change the flow of hydraulic pressure, and actuators supplying hydraulic pressure to the valves. Further, a manual valve, indexed with a driver-operated shift selector to realize port conversion, is connected to a plurality of lines to supply hydraulic pressure from the hydraulic pump to each valve and actuator.
In such a hydraulic control system, to reduce shift shock, a low level of pressure is first fed to the friction elements, then the low level of pressure (hereinafter referred as "torque pressure") is replaced with a high level of pressure (hereinafter referred as "line pressure") when the friction elements start operating.
However, when shifting from the drive D range to the reverse R range, since a plurality of the friction elements are operated, increasing inertial force, shift shock is generated.
In addition, when shifting from a fourth speed to a third speed in the drive D range, since the line pressure is directed to the friction elements which are operated in the third speed, shift shock is generated.